Several applications in processing technology require patterned thin films having properties that vary laterally along the film and/or properties that vary over the film thickness. A common way to achieve such patterned films is by deposition of a thin film on a substrate and subsequent removal of parts of the thin film of some of the locations of the substrate onto which the film was grown. In this way, parts of the substrate—where the thin film, having a specific property, has been removed—may have a first characteristic while other parts of the substrate—where the thin film having that specific property has not been removed—may have a second characteristic.
Furthermore, it is sometimes required that a second film is deposited on the same substrate after depositing and patterning the first film, such that the properties of the second film will be present at those parts of the substrate where the first film was removed. Furthermore it is possible that not the bulk property of the second film is of interest, but rather its interfacial properties at the interface with the first film. In the following, examples are given of the use of a patterned thin film in several applications.
A first example concerns the use of a patterned thin film for selecting conductive areas. Conductivity (e.g., electrical conductivity, heat conductivity, etc.) is not only a material property, but is for a certain material, also influenced by the morphology of the thin film studied. These materials include films that consist of ill-connected grains will badly conduct and films that consist of not-connected grains will not conduct at all. Selection of conductive areas using patterned thin films finds application in organic Thin Film Transistors (OTFTs), for example.
OTFTs are field-effect transistors having an organic semiconductor thin film as active semiconducting layer. Often, this thin film is a polycrystalline layer of organic small molecules, such as pentacene, oligothiophenes, phthalocyanines, and so on. In addition, organic semiconductor thin films are often unintentionally doped and therefore contain free majority charge carriers. Those properties can lead to a deficient behavior of circuits.
Generally, a TFT has an ohmic source and drain contacts to permit easy injection of charges into the semiconductor, so as to be able to sustain a high current. If a TFT works in an accumulation regime, meaning that majority charge carriers are used to form a current, several TFTs can be connected to each other not only by interconnects, but also by the common semiconductor thin film. The ohmic contacts can inject charges into the semiconductor film at any point and can form leakage currents between different TFTs working in accumulation.
The unintentional doping thus leads to a higher conductivity of the organic thin film and will increase the magnitude of the leakage currents. The dynamic range of the transistor (i.e., the ratio of the on-current to the off-current) will be limited by the higher off-current due to the stray currents in the semiconductor film outside the TFT. Using a patterned semiconductor film, such that only the active area inside the TFT can conduct current, would increase the dynamic range and reduce the leakage current in a series of different TFTs.
While solution-processable organic semiconductors can be selectively deposited only on the active areas of electronic devices, (e.g., by inkjet printing or screen printing techniques as described by Bao et al. in J. Mater. Chem. 9 (1999) 1895) other non-soluble semiconductors cannot make use of those techniques. Particularly semiconductors that are deposited as crystalline or polycrystalline films in vacuum, as the ones mentioned above, cannot make use of those techniques. They have to be patterned by other means.
Shadow masking while depositing in vacuum is one possibility described by Baude et al., yet shadow masking has a very low dimensional and alignment accuracy. Patterning of the semiconductor film after deposition by using photolithographic techniques has been demonstrated by e.g. Kane et al. in IEEE El. Dev. Lett. 21, (2000) 534. However, organic semiconductors are very sensitive to solvents and even to water, which limits the use of photoresists or else compromises at least part of the performance of the organic semiconductor. In addition, photolithography can limit applications of organic electronics where flexibility in design is required, such as circuits on demand, because they require photolithographic masks with fixed circuit designs.
Integrated shadowmasks are demonstrated by Klauk et al. in IEEE EI. Dev. Lett. 20 (1999) 289 in which a relief is fabricated on the substrate by photolithography prior to the deposition of the thin film. The relief will then break the continuity of the film during deposition. Although the organic semiconductor is not exposed to any solvents in this technique, several processing steps are still required increasing the cost of the process, and the photolithographic process again constrains the flexibility of circuit design and fabrication.
In a second example, thin film patterning is used to obtain a patterned refractive index profile to create a waveguide for light. Waveguiding of light is a prominent problem in organic lasers for example. Organic lasers often use a slab waveguide to confine the light in the organic light-emitting film, as described by Kozlov et al. in Nature 289 (1997) 362. However, only the light that is able to stimulate emission in the direction of the optical feedback structure is used efficiently in the laser. If all light emitted in the organic layer could be waveguided along the direction of the optical feedback structure, the threshold for stimulated emission may be lowered.
To waveguide light in a certain pattern on the substrate, the material in which the light is waveguided should have a higher refractive index than the surrounding material. Films with a patterned refractive index can also be used to make a distributed Bragg reflector as optical feedback in organic lasers as described by Tessler et al. in Adv. Mater. 11 (1999) 363. Moreover, it would also be useful if other properties could be spatially patterned, like the optical bandgap, the electron affinity, and the ionization potential.
A third example where patterning can be used, is in bulk hetero-junction solar cells where the interfacial properties of two materials are of importance as discussed by Yu et al. in Science, 270 (1995) 1789. The concept is to increase the interfacial area between a donor and an acceptor material, where exciton dissociation will occur. Both donor and acceptor material need to have crossed the percolation threshold, such that the hole from the dissociated exciton can be transported out of the film, and the electron can be transported out of the film.
Usually the increase of the interfacial area occurs by a random pattern in the film, usually by blending two solution-processable materials, but the bulk hetero-junction principle is not limited to this. One of the problems with the bulk hetero-junction solar cell fabricated by blending the donor and acceptor is that there is little control on the percolation of both donor and acceptor molecules. In addition, there is also little control on the interfacial area due to aggregation of the molecules. A more broad and improved application could be obtained if improved patterned films can be used.
In the above-mentioned examples, the techniques for patterning thin films are restricted to specific materials, such as solution-processable materials, which limit the flexibility of the design. Furthermore, the above-mentioned techniques for patterning thin films have a low dimensional and alignment accuracy, are less suitable to be used with organic semiconductors, increase the cost, and/or compromise at least a part of the performance of the materials in the devices made.